In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Under mobility management, when a connected UE moves within the E-UTRAN, the network needs to perform handover (HO) if the serving cell become worse than a threshold or a target cell is better than a threshold or better than the serving cell. Handover mechanism guarantees the user experience in a well-deployed network.
To greatly increase the system capacity, LTE has introduced cells with smaller coverage. Comparing to normal macro cell, these small cells transmit at smaller power and has smaller footprint. Dense deployment of small cells can bring cells close to users and therefore user can enjoy higher throughput. A network deploys macro cell and small cell in mixed ways is called a heterogeneous network (HetNet). Although small cell deployment can boost capacity, it also affects mobility. With more frequent cell change, legacy mobility mechanism would create more signaling on control plane and longer interruption on user plane, which would take away the benefit that small cell has promised.
In legacy LTE RAN, eNB change has to go through complete full handover procedure. With small cell deployment, smaller coverage would increase handover frequency. If there is no mobility enhancement, it means that every small cell change, network and UE has to go through full-scale handover procedure. The problems are twofold. First, frequency full-scale handover causes network and core network signaling storm. Every handover must go through seven signaling (between eNB/UE and eNB/eNB). If the handover were not properly finished, more signaling would be generated for RRC connection reestablishment or RRC connection request. With similar number of UE and mobility pattern, more handover is triggered within small cell deployment and therefore more signaling, which means more radio resource and processing power is consumed by mobility management. These additional signaling has a toll on network capacity. Frequent inter-node UE context transfer and core network signaling also put a pressure on X2/S1 backhaul and on MME/S-GW. Second, frequent full-scale handover causes longer data interruption. During handover, UE has to establish physical connection and reset L2 interface, which takes time. In addition, the network has to switch the data path, which has certain delay. Frequent handover means frequent data interruption, and further means higher chance of data loss. Data interruption and data loss deteriorates user experience.
To offset the negative impact, the network can group nearby small cells together to form a local (wireless) area network. Such local area network can also be linked to the overlaid macro cell. It is desirable that the mobility within the local area network has high efficiency, e.g. less signaling and small interruption. As a result, user can enjoy the high capacity that small cell deployment has promised without the burden of increased overhead. A solution for low overhead mobility in local area wireless network is sought.